Hydraulic fracturing (“fracking”) and multi-stage hydraulic fracturing are methods used to increase the economic viability of the production of oil and gas wells. Hydraulic fracturing to extract oil and natural gas involves injecting pressurized fluid and proppant through the wellbore down to and into the reservoir that contains the hydrocarbons, in order to propagate fissures in the rock layers. By this process the fissures are filled with proppant, and become the paths by which the oil and gas flow out of the rock layers and into the wellbore system. Several methods of hydraulic fracturing have been utilized.
The plug and perforate, often termed ‘plug and perf’, version of multistage hydraulic fracturing is the oldest and employs the use of wireline plugs, in conjunction with cement, to isolate between stages and wireline perforating guns to gain access to the reservoir rock.
In the plug and perforate method, casing is first installed and cemented over the reservoir zone and to surface. To initiate a frack, the frack plug is attached below perforating guns and the entire assembly is run to the bottom of the wellbore on wireline. The frack plug is set in the casing and then released. The perforating gun assembly is then pulled up to a shallower depth in the wellbore. The perforating gun charges are activated creating holes through the casing and allowing the wellbore to have fluid communication with the reservoir at the perforation point(s). The assembly is pulled out of the wellbore and the pumping of the fracture treatment into the newly perforated interval can begin. After treatment of the zone, a new plug and perforating guns are run into the wellbore to a shallower depth than the last perforations and previously stimulated zone. The process is then repeated. Typically after all zones are stimulated, the frack plugs must be milled out using a coiled tubing unit before hydrocarbon production can commence.
The consequence of the requirement for a coiled tubing unit in the plug and perforate method of hydraulic fracturing means that the horizontal and productive section of the wellbores can only be a limited length due to the frictional reach constraints of coiled tubing pipe. Recently there have been attempts to improve the multistage stage ball activated sliding sleeve ball drop style system. For example, TMK Completions Ltd. discloses an “infinite” stage system based on an electrical “counting” mechanism.
One current technology, often termed ‘ball activated sliding sleeve’ systems, in this field involves the sliding sleeve ball drop method which uses a graduated ball size functionality. This process involves first installing a production casing or liner having ports, which are covered with sliding sleeves. Each sleeve has a ball seat of a different and gradually larger size. To pump a fracture treatment, a ball is dropped into the wellbore and is pumped down to its corresponding size of ball seat where it lands and forms a seal. Pressure is increased in the upper portion of the wellbore above the seated ball until a shear member in the sleeve shears from the pressure differential, causing the now free sliding sleeve to move deeper into the wellbore and exposing a now opened port between the wellbore and the reservoir. The fracture treatment is then pumped through that port until completed. Then the next larger ball is dropped which would land and seal at the next shallowest stage. The process repeated until all desired stages have been opened and fracked. Each fracturing stage is isolated from the one below it with a slightly larger ball. The system has a finite number of stages because the size of the balls eventually increases to a size that is too large to be pumped down the wellbore. The major drawback to this method is that the number of stages is limited by the diameter of the casing, which limits the number of balls used, and in turn the number stages that can be fracked.
Other technologies related to ball-activated sliding sleeve systems are described for example in U.S. Pat. Nos. 6,907,936 and 8,863,853.
Canadian Patent Application No. 2,927,850 discloses a system for successively uncovering a plurality of contiguous ports in a tubing liner within a wellbore, or for successively uncovering individual groups of ports arranged at different but adjacent locations along the liner, to allow successive fracking of the wellbore at such locations. Sliding sleeves in the tubing liner are provided, having a circumferential groove therein, which are successively moved from a closed position covering a respective port to an open position uncovering such port by an actuation member placed in the bore of the tubing liner. Each actuation member comprises a dissolvable plug which in one embodiment is retained by shear pins at an uphole end of a collet sleeve, the latter having radially-outwardly biased protuberances (fingers) which matingly engage sliding sleeves having cylindrical grooves therein, based on the width of the protuberance. In one embodiment, when actuating the most downhole sleeve, the shear pin shears allowing the plug to move in the collet sleeve and prevent the protuberance (fingers) from disengaging. The working of the tool described in the '850 patent application require a plug of undesirably long length and profile, which makes the plug difficult to load into the wellhead at surface. It takes more time and requires extra equipment, thereby adding to the overall cost of the process. Moreover, the presence of groove in the sliding sleeve in the tool/system of the '850 patent can fill with sand and prevent an actuation member engagement.
Therefore, there is a need for a system for multistage hydraulic fracturing that is not subject to one or more limitations of the prior art.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.